1. Field of the Invention
The present application relates, in general, to data communications systems.
2. Description of the Related Art
Data is information that is in a form suitable for manipulation and/or processing in a formalized manner, such as by one or more machines. Data communications systems are systems that transmit and/or receive data through at least one data communications link (e.g., a wireless link, a wire link, or a fiber optic link).
Data communications systems often utilize what are known in the art as “modems.” “Modem” is short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over telephone lines or other communication media. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.
The operations modems perform are complex, and there are many different ways in which such operations may be done. In the early days of data communications, different vendors devised their own unique ways of doing modem operations. In order to communicate, both a transmitting and a receiving station had to use modems which understood the different schemes of the various different vendors, or communications between the two was impossible.
Over time, the industry migrated to a standards based format. Under the standards based format, various international bodies comprised of industry experts specified standards for interfaces and signals between communicating modems, so that the communicating modems did not have to know all the various unique ways in which the various vendors performed their modem operations. The idea underlying standards is that the interfaces and signal exchanges are agreed to, but the various vendors are free to provide such standard interfaces and signal exchanges however they see fit. One such standard is the V.90 modem standard of the International Telecommunication Union (ITU).
The V.90 standard is for 56-Kbps modems and was approved by the International Telecommunication Union (ITU) in February 1998. The V.90 standard was implemented to resolve a battle between two then-competing vendor-created 56 Kbps technologies: X2 from 3COM and K56flex from Rockwell Semiconductor.
The V.90 is essentially a “hybrid” modem standard, in that it specifies that continuous signal standard analog transmission modem techniques (e.g., QPSK modulation) be used to transmit data, from a “client” modem to a “server” modem (e.g., in the “upstream” direction), over the physical data transmission medium spanning the two modems, but that discrete signal Pulse Amplitude Modulation (PAM) be used to transmit data, from the server modem to the client modem (e.g., in the “downstream” direction), over the physical data transmission medium spanning the two modems.
Relatively recently, an improvement upon the V.90 standard, known as the V.92 modem standard, has been approved. In the V.92 modem standard, it has been proposed that discrete signal PAM be used to transmit data, in both the upstream and the downstream directions, over the physical data transmission medium spanning the two modems, typically over unshielded twisted pair lines (UTP).
V.92 is designed to minimize Pulse Code Modulation (PCM) codec quantization noise in both upstream and downstream directions. It employs non-equally spaced constellations to closely match PCM codec quantization levels (e.g. μ-law codec in North America and A-law code in Europe), on an attempt to minimize codec quantization noise. For modern PCM modem design, PCM codec quantization noise is considered one of the dominant impairments that affect modem performance (bit error rate).
There exist several well-known side effects of the use of non-equally spaced constellations, such as increase of peak-to-average (power) ratio (PAR), reduced minimum distance for a given average power and so on. However, quantization noise reduction due to use of PCM (codec-matched) constellation levels significantly outweighs the above “side” effects. Therefore, both V.90 and V.92 modems yield much high data rates than the conventional modem technologies such as V.34 when transmitting over a PCM channel.
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